The difference between the fluids is the temp rating. You shouldn't mix or use opened container brake fluids (unless its known to be free of contaminates) that havn't been stored properly. The blue dot 4 is a higher temp, purple is a synthetic grade 4, the dot 3 is not.

Flush and fill with one or the other of at least the grade the car requires .

DOT4 and DOT3 are both polyethylene glycol based, which is a synthetic polymer. There's no such thing as a mineral based automotive brake fluid. It'd be similarly redundant to market water as inorganic.

DOT4 increases the boiling point over DOT3 by adding borate esters. That's it. If the bulk of the fluid had performance characteristics sufficiently beyond the minimum required by FMVSS, adding a small amount of DOT3 is nothing to sweat about. Obviously the highest boiling point possible is desirable for safety reasons, but you'd have to be driving like a crazy person to get into trouble.

DOT4 is cheap to produce, so fluids with DOT3 maximum performance don't make much sense. Some manufacturers have resorted to just packing DOT4 into bottles marked DOT3/DOT4 so the target market will not ask questions.

What is important, however, and was mentioned before, is care in handling PEG based brake fluids (everything but the silicone DOT5). Always use new bottles, because the oil immediately starts absorbing moisture, hence the requirement to change it every year or two to maintain the boiling point.

agree with responses #4 and #5- DOT3 and DOT4 brake fluids are yes compatible to mix together. But do not mix either with DOT5.
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[EDIT after reading response #10- agree 100%. DOT3 and DOT4 are not equivalent (as explained above), but they are compatible to mix. So, if DOT3 was introduced into a system containing DOT4, no need for concern about gelling or chemical reactions etc. but the overall boiling point of the system will be reduced by the DOT3 accordingly.]
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imho- OP didn't shoot himself in the foot by mixing a bit of DOT3 with the majority of DOT4 in his car. However.... if it was an old bottle of DOT3 that had been previously opened and capped, but sat on the shelf for a long time... that might have been a foot wound, not because the DOT3 is incompatible, but rather because the DOT3 might be loaded with moisture.

Annnddd just to beat the dead horse. All brake fluids are synthetic and come in two flavors, PEG and silicone. Dot 3 & 4 are completely compatible but you should assume a performance in between them when you mix them.

On a car that hasn't had its brake fluid flushed in a while I like to start out with DOT3, leave it in for a week, then re-flush with DOT4 to get all the crap out.

-J

I agree. When we got our 04, the brake fluid was pretty nasty(probably original).When I sucked out the old fluid, there was still black goop stuck to the sides of the tank. That was only a few years too since we got it in 2010.

It says to change it every 2 years in the manual and I can see why. I probably should just flush it once a year for a few bucks in costs in fluid. A rock hard brake peddle is nice too!

If DoT 4 absorbs water more readily, perhaps that is why the 2-year replacement is required by VW. All the American cars manufacturers seem to specify specify DoT 3 and have no fluid change requirement.